The proposed research is aimed at obtaining the date required for the development of therapeutic applications of the ionophorous antibiotics, especially to the treatment of heart disease, as well as for applications of these compounds in the study of basic processes in cells, especially ion transport in cell membranes. The study is centered on X-537A and monensin, both of which can be given a spin label without destroying the biological function, while for X-537A the circular dichroism also provides a method for elucidating the ion- binding processes. For the interactions of these antibiotics with each of the alkali and alkaline-earth metal ions as well as a few paramagnetic transition metal ions, it is planned to investigate the stoichiometries, equilibrium constants, and, as far as possible, rate constants. Preliminary measurements are described which demonstrate the power of the CD measurements for elucidating the complexes of X-537A. For monensin, the interactions will be studied either using the spin-label probe or by competition with X-537A. The rates are to be studied as far as possible by the effects upon the EPR spectra of the spin labels, although methods based on the effect of the NMR spectra in binding paramagnetic ions and methods based upon the fluorescece of X-537A may also be applied. These studies are to be carried out in various solvents such as hexane, which serves as a model for the interior of a membrane, and methanol. As guided by the results in these homogeneous systems, the studies will be extended to phospholipid vesicles.